1. Field of the Invention
The present invention relates to an equalization method for a signal such as communication data, in particular to an adaptive equalization method for a data receiver circuit engaged in a data exchange between boards or chassis mounted with LSI for example and to an adaptive equalizer circuit for reducing a distortion of received data influenced by a transmission distortion, or a disturbance, occurring in a transmission path.
2. Description of the Related Art
When transmitting and receiving data by way of a transmission line with a large loss, an equalizer circuit is generally used at a receiving end in order to compensate for the loss. FIG. 1 describes a concept of adaptive equalization. As shown by FIG. 1, when an eye pattern in an output signal coming out of transmission line transforms with a temperature change in the transmission line for example, an equalizer circuit comprised at the receiving end performs an adaptive equalization for securing a data amplitude of the received signal, thereby making it possible to secure a sufficient data amplitude for enabling a data judgment circuit to judge the data. When exchanging data by way of a transmission line with a large loss, such as a low cost transmission line, the adaptive equalization for carrying out an adjustment of data amplitude according to the characteristic of the transmission line is an indispensable technique.
FIG. 2 shows a configuration of an analog derivative equalizer as a representative example of equalizer circuit. This equalizer circuit comprises a one time derivative element (s) and a two times derivative element (s2) in order to emphasize a change in input signals. In the adaptive equalization, it is necessary to secure a sufficient magnitude of data amplitude as an input to the data judgment circuit by adjusting the coefficients A0, A1 and A2 of respective filters constituting the equalizer circuit. In other words, the adaptive equalization is a technique to secure a receiver signal amplitude so as to satisfy a bit error rate required at a receiving end by adjusting the coefficients of the respective filters.
In FIG. 2, a control for accomplishing an adaptive equalization will be such that there is no correlation between an input amplitude Fi (i=0, 1, 2) and an amplitude error e, where an output of equalizer circuit 101 being comprised between a transmission line 100 and data judgment circuit 102, that is, a data amplitude of an input to the data judgment circuit 102 is y, an expected amplitude after an equalization is d, and an amplitude error is then e=d−y.
FIG. 3 shows an impulse response in the analog derivative equalizer shown by FIG. 2. An input to the equalizer circuit 101, that is, an output from the transmission line 100 has a mild rise and a large inter-signal interference (ISI) component. Comparably, an output of the equalizer circuit 101 has a steep rise with the change in signal being emphasized and also a smaller ISI.
FIG. 4 shows a block diagram of conventional example configuration of adaptive equalization method. In FIG. 4, a transmitted data from a transmission circuit 105 by way of a transmission line 100 is received by a receiver circuit 106. The inside of the receiver circuit 106 comprises an equalizer circuit 101 which is equivalent to the analog derivative equalizer shown by FIG. 2, a data judgment circuit 102, a de-multiplexer (1 to N) & buffer 107 for providing one input (of N bits) to an adaptive equalizer control circuit 110 while receiving an output of the data judgment circuit 102 and an AD converter 108 for AD-converting the output of the equalizer circuit 101, that is, the input to the data judgment circuit 102.
And the adaptive equalizer circuit control 110 is configured for carrying out a convolution operation by using the outputs of a matrix 111, which is the matrix reflecting a characteristic of each filter constituting the transmission line 100 and that of equalizer circuit 101, and de-multiplexer & buffer 107; predicting the signal amplitudes F0, F1 and F2 at the input to a gain amplifier for each filter which is an internal node of the equalizer circuit 101; computing the coefficients A0, A1 and A2 of the respective filters of the equalizer circuit 101 by using the aforementioned result of prediction and amplitude error e; and having each coefficient adjusted.
The inside of the adaptive equalizer control circuit 110 comprises a convolution operation unit 112; a selector 113 for selecting one bit of N-bit output from the de-multiplexer & buffer 107; an amplifier 114; a subtracter 115 for calculating an amplitude error “e” which is expressed as e=d−y, where a predicted amplitude “d” as the output of the amplifier 114, and the input data amplitude “y” to the data judgment circuit 102 as the output of the AD converter 108; three multipliers for operating each filter coefficient included in the equalizer circuit 101 by using the above described amplitude error “e”, signal amplitudes F0, F1 and F2; three of step size parameters ssp as a variable for determining a convergence time constant of adaptive equalization loop; and three integrators.
Note that, in FIG. 4, a calculation of amplitude error e needs to know where a data relating to the output of the AD converter 108 is located in the output of the de-multiplexer & buffer 107. The buffer of the de-multiplexer & buffer 107 is for locating the data. The locating on the side of receiver circuit 106 is not necessarily required, but the assumption here is done thereby just for convenience. Since the locating is done by a known technique and therefore a detailed description thereof is omitted herein. Furthermore in FIG. 4, another assumption is that the adaptive equalizer control circuit 110 is constituted by a logic circuit, for which the AD converter 108 is equipped on the side of the receiver circuit 106.
FIG. 5A, FIG. 5B and FIG. 5C shows a result of simulation indicating an arithmetic logical operation (simply “operation” hereinafter, unless otherwise noted) of adaptive equalization loop, where the predicted amplitude d of the input data to the data judgment circuit 102 is set at ±0.1Vpp. Despite that the temperature at the transmission line varies from minus 20 to plus 85 degrees Celsius, one can understand from FIG. 5A, FIG. 5B and FIG. 5C that the average of input data amplitudes is approximately ±0.1Vpp, equal to the predicted amplitude, hence securing an adequate data amplitude. Understanding also is that the coefficients A0, A1 and A2 of the respective filters constituting the equalizer circuit respectively converges around 5000 ns as shown by the uppermost charts and that the square average of the amplitude error has become smaller as shown in the middle.
Such an adaptive equalization method using the analog derivative equalizer is seen in the following reference document in which FIG. 8, 39 are applicable to the conventional configuration shown by FIG. 4 herein.
[Non-patent document 1] Jan W. M. Bergmans: DIGITAL BASE BAND TRANSMISSION AND RECORDING, 8.5, Kluwer Academic Publishers (1996)
The conventional configuration shown by FIG. 4, however, has been faced with the problem that there is a limitation in a range of tracking the characteristic changes due to temperature and/or secular changes of a transmission line because the content of the matrix, as the matrix reflecting the characteristics of transmission line and respective filters constituting the equalizer circuit, is fixed. Particularly, if a characteristic of transmission line changes greatly, a use of given matrix will no longer be able to find appropriate values of the coefficients A0, A1 and A2 of the respective filters constituting the equalizer circuit. Such has been a problem.
In such events, the conventional configuration requires a change in the matrix, that is, needs to prepare specific matrices in response to temperature and/or secular changes of the transmission line, and uses a suitable matrix for each range of temperature for example. A preparation of such a plurality of matrices and a grasp of tracking ranges for the characteristic of the transmission line for each matrix demand a series of works in need of vast amount of man-hours, hence becoming a large obstacle in an actual operation of an equalizer circuit. Such has been the problem.